The Secular Evolution of a Close Ring-Satellite System: The Excitation of Spiral Density Waves at a Nearby Gap Edge

TL;DR

This paper models how a small satellite within a planetary ring excites spiral density waves at a nearby gap edge, affecting the ring's structure and the satellite's orbital eccentricity, with implications for Saturn's moons Pan and Daphnis.

Contribution

It derives the dispersion relation for spiral density waves excited by a satellite in a planetary ring and analyzes their impact on satellite eccentricity damping.

Findings

Long-wavelength spiral density waves can be excited at gap edges.

These waves can damp satellite eccentricity over time.

The damping rate is comparable to resonance-driven eccentricity changes.

Abstract

The Lagrange planetary equations are used to study to secular evolution of a small, eccentric satellite that orbits within a narrow gap in a broad, self-gravitating planetary ring. These equations show that the satellite's secular perturbations of the ring will excite a very long-wavelength spiral density wave that propagates away from the gap's outer edge. The amplitude of these waves, as well as their dispersion relation, are derived here. That dispersion relation reveals that a planetary ring can sustain two types of density waves: long waves that, in Saturn's A ring, would have wavelengths of order 100 km, and short waves that tend to be very nonlinear and are expected to quickly damp. The excitation of these waves also transports angular momentum from the ring to the satellite in a way that damps the satellite's eccentricity e, which also tends to reduce the amplitude of subsequent waves. The rate of eccentricity damping due to this wave action is then compared to the rates at which the satellite's Lindblad and corotation resonances alter the satellite's e. These results are then applied to the gap-embedded Saturnian satellites Pan and Daphnis, and the long-term stability of their eccentricities is assessed.